Here’s an app note (PDF) from Fairchild Semiconductor on how to solve audible noise problem on their power switch applications.
In general, switched mode power supplies do not generate audible noise when they operate at constant ultrasonic frequencies (>20kHz). However, some switched mode power supplies can produce audible noise at certain load conditions. Most Fairchild Power Switches are designed to enter into burst switching operation at light load conditions to reduce standby consumption, which can cause audible noise when the fundamental frequency of the burst switching bundles is in the range of human hearing.